The present invention generally relates to an effusion cell assembly and, in particular, relates to such an effusion cell assembly having a self-supporting filament.
In the general field of semiconductor manufacturing one of the most technologically sophisticated systems is known as the molecular beam epitaxial growth system (hereinafter referred to as MBE or MBE system). In very simplified terms an MBE system is one in which thermally excited atoms or molecules of one or more materials, for example, a semiconductor dopant, are produced in effusion cells and, as beams, bombard a semiconductor substrate. By bombarding a substrate in an accurate and selective fashion, well-defined layers of various compositions are formed on the substrates. These well-defined layers then serve as the essential structure for the fabrication of semiconductor devices. The thickness of these layers can, by, inter alia, a computer controlled mechanism, be very accurately controlled, thus resulting in well-defined structures. As those skilled in the semiconductor art will recognize, a critical factor in the fabrication of any semiconductor device on a substrate is the depth and composition of the dopant profile of the layered structure. Ideally, in most instances, the composition should be uniform throughout a particular layer. MBE systems appear capable of producing structures consisting of well-defined and abrupt interfaces. It is in furtherance of this goal that the present effusion cell assembly is directed.
As stated, one of the most critical components of an MBE system is the effusion cell assembly. In general, an effusion cell is the source of the atomic or molecular beam. Usually, a material is placed in the effusion cell assembly, which is effectively a crucible formed of a refractory material, and heated to a temperature at which a beam of atoms or molecules is emitted therefrom. The beam fluxes, i.e., the cross sectional density of atoms or molecules as well as the purity thereof impinging upon the substrate directly determines the composition growth rate for each molecular or atomic layer of the structure as well as the electrical characteristics thereof.
Although conceptually an MBE system appears straightforward, in actual practice many factors must be considered. For example, the materials used in the construction of the effusion cell assembly are critical because the cell is required to operate at rather high temperatures (about 1500.degree. C.) and under an ultra high vacuum (usually about 10.sup.-10 Torr). Consequently, the materials chosen must be as free of impurities as possible to avoid out-gassing or decomposition, either of wich would severely contaminate the beam flux impinging on the semiconductor substrate.
The primary element of the effusion cell assembly is, of course, the filament which is electrically heated and radiantly heats the crucible in which the material to be evaporated is contained.
Conventional filaments for use in MBE effusion cell assemblies are rather complex arrangements and commonly use a tantalum wire, or ribbon, arranged on a pyrolytic boron nitride (pBN) support system. For example, one assembly involves weaving a tantalum wire through a plurality of perforated pBN discs. Another such assembly involves coiling a tantalum wire about a pBN tube. Although other assemblies are known, two known drawbacks are common to all such assemblies. First, tantalum filaments have a low emissivity and thus the required operating temperature of the filament is considerably higher than the crucible temperature. Second, such assemblies are inherently expensive due to the complex structure thereof.